1. Field of the Invention
The invention relates to a transflective LCD device, more particularly to a transflective LCD device using different common voltages in the transmissive and reflective regions to present the same gray scale performance on the transmissive and reflective regions.
2. Description of the Prior Art
A color liquid crystal display (LCD) panel comprises two transparent substrates and a liquid crystal layer interposed therebetween. Normally, the commonly used liquid crystal of the TFT LCD device is TN (Twisted nematic) liquid crystal, which is nematic. The liquid crystal molecules are arranged with regularity in one-dimension. All the long axes of the clubbed liquid crystal molecules are correlatively arranged in parallel according to a specific direction. The nematic liquid crystal easily flows due to its low viscosity because this molecule easily flows along the direction of its long axis.
As the structure of liquid crystal molecule is anisotropic, the induced photo-electronic effect will differ according to its arranging direction. In other words, the photo-electronic properties of liquid crystal molecule such as the dielectric permittivity or the refractive constant are anisotropic, too. Thus, the different gray scales displayed on the LCD can be formed by using the above matters to change the intensity of the incident light. For example, the dielectric permittivity can be divided into two vectors: ε∥ (in parallel with the long axis of liquid crystal molecule) and ε⊥ (vertical to the long axis of liquid crystal molecule). If ε∥>ε⊥, the dielectric anisotropy of the liquid crystal is called as positive and If ε∥<ε⊥, the dielectric anisotropy of the liquid crystal is called as negative. When a voltage is applied on the liquid crystal molecule, which will rotate parallel to or vertical to the electric field due to the positive or negative value of the dielectric anisotropy for permitting the light rays to pass through the liquid crystal or not. Now, the dielectric anisotropy of the TN type liquid crystal used in the TFT-LCD is almost positive.
FIG. 1A shows the arrangement of the positive type liquid crystal that is not applied the voltage. Presently, the pixel electrode 12 and the common electrode 13 are not applied voltage or the voltage difference on the liquid crystal layer 11 is zero so that the liquid crystal molecules 14 are arranged parallel to each other. Thus, the light can passes through not only the transmissive region but also the reflective region so as to be displayed on the screen of LCD. On the contrary, when a voltage is applied to the liquid crystal layer 11, the liquid crystal molecules 14 begin to rotate and are not arranged parallel to each other. Then the liquid crystal molecules 14 will arrange vertically to the pixel electrode 12 and the common electrode 13 until the voltage achieves a specific value V1 as shown in FIG. 1B. Thus, the light cannot pass through not only the transmissive region but also the reflective region, so it cannot be displayed on the screen of LCD.
FIG. 2A shows a transmissive rate to applied voltage curve (T-V Curve) and a refractive rate to applied voltage curve (R-V Curve). As shown in FIG. 2A, we can understand that the transmissive rate or the refractive rate is decreased while the applied voltage is increased. Therefore, the intensity of incident light rays can be varied by means of the above properties in order to display different gray scales on the screen of LCD. The conventional transflective liquid crystal display device is described with reference to FIG. 2B. The pixel electrode comprises a transmissive electrode 21 and a reflective electrode 22 that are connected electrically each other. The common electrode 23 is a transparent conductive layer formed in the transmissive and reflective regions. When a voltage is applied to the pixel electrode and the common electrode, the external voltage applied in the transmissive region is the same as that in the reflective region. As the T-V curve does not overlap the R-V curve as shown in FIG. 2A, the measured transmissive rate and the reflective rate are different when a fixed applied voltage is provided. It causes the gray scale displayed in the transmissive region is different from the gray scale displayed in the reflective region. In other words, the gray scale displayed on the screen of LCD by interior light source through the transmissive region (transmissive mode) is different from the gray scale displayed on the screen of LCD by exterior light source through reflective region (reflective mode). For example, the LCD presents blue under transmissive mode, while the LCD presents pale blue under reflective mode. The users will query the quality of the product.